In most animal species, mitochondrial DNA is inherited solely from the mother, unlike the nuclear DNA that is inherited from both parents. Why and how fathers’ mitochondrial DNA disappears from the zygotes remains a mystery to biologists. To solve the mystery, Professor Byung-Ho Kang at CUHK and Prof. Ding Xue’s group at the University of Colorado Boulder examined the mitochondria in the sperm of C. elegans using electron tomography. They found that the sperm mitochondria started self-degradation once a sperm penetrates an egg.